1. Field of the Invention
This invention relates to an abrasive blasting apparatus which utilizes a blast of abrasive particles for surface treatment of a workpiece. The present invention also relates to a die or mold finishing apparatus which uses such an abrasive blasting apparatus the finish inner cavity surfaces of a die or mold as a workpiece.
For simplicity, the term "die" is used in the following description to mean either one of a die or a mold.
2. Description of the Prior Art
An abrasive blasting apparatus utilizes a blast of abrasive particles entrained in a high-speed air flow, and is widely used for performing surface treatments (e.g. ground conditioning, Butler finishing, and etc.) of metal workpieces or the like or for removing flashes or burrs of formed resin or metal products. The hardness, grain size and/or other characteristics of the abrasive particles are suitably selected depending on the nature of the workpiece and/or the purposes of the blasting operation.
Typically, a suction-type abrasive blasting apparatus comprises a blast chamber, a blast nozzle arranged in the blast chamber, and an abrasive container for storing particles of an abrasive material. The blast nozzle is connected to an external air source through an air supply hose on one hand, and to the abrasive container through an abrasive supply hose on the other hand. When the air source is actuated to pass a high-speed air stream through the blast nozzle, the abrasive particles gravitationally introduced into the blast nozzle are drawn into the high-speed air stream for blasting under the assistance of the suction force generated by the air stream.
For improving the quality of surface finishing, it is often necessary to perform the surface finishing at least in two successive stages which include rough finishing (primary finishing) and final fine finishing (secondary finishing). Relatively hard, coarse abrasive particles are used for the rough finishing, whereas relatively soft, fine abrasive particles are used for the final fine finishing.
The conventional abrasive blasting apparatus described above is not suitable for performing such a two-stage surface treatment. Specifically, since the conventional apparatus utilizes a single abrasive material, it is necessary to entirely replace the abrasive material by a new batch of a different abrasive material for the secondary finishing after completing the primary finishing. Obviously, such a change of abrasive materials is very troublesome and will result in low process efficiency. Further, if a portion of the abrasive material for the primary finishing is unexpectedly left in the system (which is quite likely) and mixed with the later loaded abrasive material, the quality of the secondary finishing will be inevitably deteriorated.
It is also conceivable to use two different abrasive blasting apparatuses for separately performing the two kinds of surface treatments. However, such a solution leads to an unacceptable increase of equipment cost.
The requirement for a two-stage surface treatment is particularly strict with respect to dies which are used for forging, die casting, resin molding, and the like. The strict requirement is attributable to the fact that a recent development in electrical discharge machining has made it possible to obtain a die which has a very complicated inner cavity. Indeed, due to the complication of the inner cavity, two-stage surface finishing of the die need be performed primarily by intensive manual operations. Thus, there is a strong need for automating the two-stage surface finishing operation with respect to dies.